FOUNDATIONS OF QUANTUM MECHANICS

I. 2. INCOMPLETENESS AND LOCALITY 15
by ‘influence’ the ‘defining effect’ of the context of measurement. The experimental circumstances
define what you may call physical reality. Physical reality is not defined by experiments you could do,
as is the case according to EPR, but exclusively by experiments you actually do. Under circumstances
as described in the EPR experiment this ‘defining effect’ of the experimental setup also reaches parts
of the system with which the measuring apparatus has no physical interaction.
A distinct difference between Einstein and Bohr is that Einstein wants to visualize reality independent
of observation, whereas Bohr is satisfied with complementary pictures of which the applicability
always remains dependent on the chosen measurement setup. In 1955 Einstein says (Fine 1986, p.95)
It is basic for physics that one assumes a real world existing independently from any act
of perception. But this we do not know. We take it only as a programme in our scientific
endeavors. This programme is, of course, prescientific and our ordinary language is
already based on it.
And concerning the EPR situation he says (Schilpp 1949, p. 85)
But on one supposition we should, in my opinion, absolutely hold fast: the real factual
situation of the system S 2 is independent of what is done with the system S 1 , which is
spatially separated from the former.
Bohr’s conceptions concerning physical reality are much more difficult to characterize. According
to him there is no independent reality of which the physical theory would have to give an unambiguous
representation. He writes (Schilpp 1949, p. 211)
Thus, a sentence like “we cannot know both the momentum and the position of an atomic
object” immediately raises questions as to the physical reality of two such attributes of the
object, which can be answered only by referring to the conditions for the unambiguous
use of space - time concepts, on the one hand, and dynamical conservation laws, on the
other hand.
An exhaustive description of reality must always use concepts which themselves remain dependent
on mutually excluding contexts. Bohr says (A. Petersen 1963, p. 11)
The word ‘reality’ is also a word, a word which we must learn to use correctly.
He constantly emphasizes the restricted applicability of our physical concepts, which makes the link
between description and reality very complicated. Petersen mentions (ibid., p. 12)
When asked whether the algorithm of quantum mechanics could be considered as somehow
mirroring an underlying quantum world, Bohr would answer, “There is no quantum
world. There is only an abstract quantum physical description. It is wrong to think that
the task of physics is to find out how nature is. Physics concerns what we can say about
nature.”